Sentence Correction

@Ronald, Can you please also mention the source of the question → This way you will attract more answers, quality one.?

Hello!

Thank you for your question. Since you asked mainly about the differences between answers C and D, let's stick to just those. We know the other three options are wrong because they use phrases like "with such rapidity" and "in such rapidity," which are both over wordy and awkward.

Now that we've narrowed it down to C and D, let's look at each one and why D is the better answer:

The number of mountain gorillas is declining with such rapidity that the population is one-half in the twenty years between a count made by George Schaller in 1960 and the one made by Dian Fossey in 1980.

(C) so rapidly the population divided in half
When we say " the population divided in half," it suggests that all of the mountain gorillas were still alive, but split into two groups at some point. That's not what the writer is trying to say - he/she means the population is half the size it used to be because half of them died. Therefore, this one is wrong.

(D) so rapidly that the population was halved
This one is right because it clears up the extra wordiness by simply saying "so rapidly," and by saying the population "was halved," it's clear the writer is saying that half of the population died, which is true.

I hope this helps! I'm available if you'd like any follow up.